communication systems for second year ECE

EC3491 - COMMUNICATION SYSTEMS

1. Communication Systems By, Dr. D. Veera Vanitha , Associate Pofessor /ECE, School of Engineering, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore

2. Communication A process by which information is exchanged between individuals through a common system of symbols, signs, or behavior . Examples: Telephone TV Radio Cell phone Satellite etc., 2

3. Communication Process Transmitter Message The media Receiver Noise 3

4. What is Signal? It is representation of physical quantity (Sound, temperature, intensity, Pressure, etc..,) which varies with respect to time or space or independent or dependent variable or It is single valued function which carries information by means of Amplitude, Frequency and Phase. Example: voice signal, video signal, signals on telephone wires etc. 4

5. Classification of Signals One Dimensional or 1-D Signal: If the signal is function of only one variable or If Signal value varies with respect to only one variable then it is called “One Dimensional or 1-D Signal” Examples: Audio Signal, Biomedical Signals, temperature Signal etc.., in which signal is function “time” 5

6. Two Dimensional or 2-D Signal: If the signal is function of two variable or If Signal value varies with respect to two variable then it is called “Two Dimensional or 2-D Signal” Examples: Image Signal in which intensity is function of two spatial co-ordinates “X” & “Y” i,.e I (X,Y) and also time “t” i.e v( x,y,t ) 6

7. Three Dimensional or 3-D Signal: If the signal is function of three variable or If Signal value varies with respect to three variable then it is called “Three Dimensional or 3-D Signal” Examples: Video Signal in which intensity is function of two spatial co-ordinates “X” & “Y” 7

8. Types of Signal Types of Signal with respect to nature of the signal Continuous Time Signal (CTS) or Analog Signal : If the signal values continuously varies with respect to time then it is called “Continuous Time Signal (CTS) or Analog Signal “. It contains infinite set of values. Discrete Time Signal (DTS): If signal contain discrete set of values with respect to time then it is called “Discrete Time Signal (DTS)”. It contains finite set of values. Sampling process converts Continuous time signal in to Discrete time signal. Digital Signal: If the signal contains only two values then it is called “Digital Signal”. 8

9. Time Domain representation Changes in amplitude with respect to time Frequency Domain representation Changes in amplitude with respect to frequency 9

10. What is Modulation Modulation In the modulation process, some characteristic of a high-frequency carrier signal ( bandpass ), is changed according to the instantaneous amplitude of the information (baseband) signal. Why Modulation Suitable for signal transmission (distance…etc) Multiple signals transmitted on the same channel Capacitive or inductive devices require high frequency AC input (carrier) to operate. Stability and noise rejection 10

11. About Modulation 11 Application Examples broadcasting of both audio and video signals. Mobile radio communications, such as cell phone. Basic Modulation Types Amplitude Modulation: changes the amplitude. Frequency Modulation: changes the frequency. Phase Modulation: changes the phase

12. Amplitude Modulation The amplitude of high-carrier signal is varied according to the instantaneous amplitude of the modulating message signal m(t). 12

13. Modulation Index of AM Signal 13 Modulation index k is a measure of the extent to which a carrier voltage is varied by the modulating signal. When k=0 no modulation, when k=1 100% modulation, when k>1 over modulation.

14. 14 In frequency domain the AM waveform are the lower-side frequency/band ( f c - f m ), the carrier frequency f c , the upper-side frequency/band ( f c + f m ). The sidebands contain the information and have maximum power at 100% modulation. P t = P c (1 + k 2 /2) P t =Total transmitted power (sidebands and carrier) P c = Carrier power

15. Double Sideband Suppressed Career AM 15 Full-carrier AM is simple but not efficient. Removing the carrier before power amplification allows full transmitter power to be applied to the sidebands. Removing the carrier from a fully modulated AM systems results in a double-sideband suppressed-carrier transmission

16. Single Sideband AM 16 The two sidebands of an AM signal are mirror images of one another. As a result, one of the sidebands is redundant Using single-sideband suppressed-carrier transmission results in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC

17. Vestigial Sideband AM 17 The process where the “vestige” part of a signal is modulated alongside one sideband.

18. Angle Modulation 18

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21. Random process 21

22. Probability 22

23. Noise 23

24. Sources of Noise 24

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26. Pre-emphasis works by boosting the high-frequency portion of the signal. This compensates for the high-frequency loss in the cable. De-emphasis works by cutting the low-frequency portion of the signal. This may be coupled with an increased transmit voltage. 26

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28. Quantization Quantization is a process to convert the continuous analog signal to the series of discrete values. A quantizer is a device known to perform the quantization process. The function of quantizer is to represent each level to the fixed discrete finite set of values. The signals during transmission over long distances suffer from noise and interference. To overcome this, the quantization process creates a signal that is approximately equal to the message signal. Types of Quantization There are two types of Quantization, uniform Quantization and non-uniform Quantization. 28

29. Pulse Modulation Techniques PAM- In pulse amplitude modulation, the amplitude of the pulsed carrier wave is varied according to the modulating signal value. PWM- In pulse width modulation, the width of pulsed carrier is varied as per the value of the modulating signal. PPM- In pulse position modulation, the position of the pulsed carrier is varied as per value of modulating signal. 29

30. Multiplexing The process of converting many signals into one signal over a shared medium is called Multiplexing. If the analog signals are multiplexed it is called analog multiplexing and if the digital signals are multiplexed it is referred to as digital multiplexing. The reversible process used for the extraction of a number of channels from a single signal, implemented at the end of multiplexer or receiver is known as de-multiplexing. This process is made possible with de-multiplexer and it’s popularly known as DEMUX. Demux splits the signal into its related or component signals. So it has only single input and many outputs. 30

31. Digital Techniques Pulse Code Modulation (PCM) Pulse Code Modulation is the technique used for reworking analogue signals into digital signals. PCM has a good or sensible signal-to-noise ratio. For transmission, Pulse Code Modulation wants high transmitter bandwidth. PCM technique is split into three elements, initial is the transmission at the provision end, second regeneration at the transmission path and conjointly the receiving end.  31

32. Delta Modulation(DM) Delta modulation is an analog to digital and digital to analog signal conversion technique. Delta modulation is employed to realize high signal to noise ratio. It uses one bit PCM code to realize digital transmission of analog signal. With delta modulation, instead of transmit a coded illustration of a sample solely one bit is transmitted, that merely indicates whether or not the sample is larger or smaller than the previous sample. it’s the best type or simplest type of Differential Pulse Code Modulation. Delta modulation signal is smaller than Pulse Code Modulation system. If signal is large, the next bit in digital data is 1 otherwise 0.  32

33. Differential pulse-code modulation (DPCM) is a signal encoder that uses the baseline of pulse-code modulation (PCM) but adds some functionalities based on the prediction of the samples of the signal . The input can be an analog signal or a digital signal. 33

34. Channel Coding Theorem 34 The idea of using a channel coding scheme is to recover from errors that occur during transmission over the communication channel.

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45. Convolutional Codes Unlike block codes that operate on block-by-block basis, the convolutional codes are based on message bits that come in serially rather than in blocks. The structure of a generic convolutional encoder can be written in a compact form of (n, k, L) ; n= no of streams, or no of modulo-2 adder; k=number of group of shift register; L=number of state of the shift register The encoder takes an L-bit message sequence to produce a coded output sequence of length n(L+M) bits, where n=no of stream of the encoded bits, M=no of bits of message sequence The encoder is implemented by a tapped shift register with L+1 stages. 45

46. 46 c j The message bits in the register are combined by modulo-2 addition to form the encoded bit, c j C j = m j-L .g L + …..+ m j-1 .g 1 + m j .g =

47. To provide the extra check bits for error control, the encoded bits from multiple streams are interleaved. For example, a convolutional encoder with n=2 ( ie two streams of encoded bits) 47 c’ j c’’ j Encoded bits from stream 1, c’ j = m j-2 + m j-1 + m j Encoded bits from stream 2, c’’ j = m j-2 + m j C= c’ 1 c’’ 1 c’ 2 c’’ 2 c’ 3 c’’ 3 ………..

48. Each stream may be characterised in terms of its impulse response ( the response of that stream to a symbol 1 with zero initial condition). Every stream is also characterised in terms of a generator polynomial, which is the unit-delay transform of the impulse response Let the following generator sequence denotes the impulse response of the i- th path 48 The generator polynomial of the i -th path is given by Where D denotes the unit-delay variable

49. Convolutional Codes – Example A convolutional encoder with two streams of encoded bits with message sequence 10011 as an input 49 c’ j c’’ j First, we find the impulse response of both streams to a symbol 1. Impulse response of stream 1 = (1 1 1) Impulse response of stream 2 = (1 0 1) Then, write the corresponding generator polynomial of both streams

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52. Convolutional Codes – Code Tree, Trellis and State Diagram Code Tree 52

53. Trellis 53

54. State Diagram 54

55. Maximum Likelihood Decoding of a Convolutional Code 55

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60. The Viterbi Algorithm The equivalence between maximum likelihood decoding and minimum distance decoding implies that we may decode a convolutional code by choosing a path in the code tree whose coded sequence differs from the received sequence in the fewest number of places Since a code tree is equivalent to a trellis, we may limit our choice to the possible paths in the trellis representation of the code. Viterbi algorithm operates by computing a metric or discrepancy for every possible Path in the trellis The metric for a particular path is defined as the Hamming distance between the Coded sequence represented by that path and the received sequence. For each state in the trellis, the algorithm compares the two paths entering the node And the path with the lower metric is retained. The retained paths are called survivors. The sequence along the path with the smallest metric is the maximum likelihood Choice and represent the transmitted sequence The Viterbi Algorithm -example In the circuit beliow , suppose that the encoder generates an all-zero sequence and that the received sequence is (0100010000…) in which the are two errors due to channel noise: one in second bit and the other in the sixth bit. We can show that this double-error pattern is correctable using the Viterbi algorithm. 60

61. Digital Modulation Scheme 61

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63. M- ARY Modulation 63

64. Inter Symbol Interference 64

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